I suggest you ...

Allow custom default LifetimeManagers

One of the things I like most about Unity, is that Unity can build object trees, even if not all types used in the object tree are explicitly registered.
However, if I want to use a custom default LifetimeManager, I do have to register all types with the custom LifetimeManager, because Unity does not support a custom default LifetimeManager.
So, the request is simple: make the default LifetimeManager type configurable.